Environmental factors affecting diffuse nitrate pollution in the major aquifers of central Spain: groundwater vulnerability vs. groundwater pollution

Three Tertiary aquifers and one Mesozoic aquifer located in the two neighbouring Autonomous Communities of Madrid and Castilla-La Mancha (central Spain) were selected to study the environmental factors affecting groundwater pollution by nitrate. The majority of the territory was subject to a very high degree of human impact, with it being difficult to identify which potential sources of nitrogen were responsible for the observed cases of groundwater nitrate pollution. The objectives were: (1) to obtain a better understanding of nitrate distribution in the aquifers during a period of 25 years; (2) to determine the relative contribution of several environmental parameters (including land use) that are often used in groundwater vulnerability assessment using principal components analysis (PCA); (3) to make a critical review of the current criteria used for the designation of nitrate vulnerable zones (NVZs; Directive 91/676/EEC) in this territory; and (4) to discuss the concept of groundwater vulnerability to nitrate vs. groundwater nitrate pollution. Contour maps of nitrate contents in the four aquifers from 1985 to 2010 showed a persistent problem of nitrate pollution affecting wide areas of the Tertiary aquifers (subjected to a high risk of anthropogenic impact), whereas the Mesozoic aquifer remained unpolluted (protected by non-polluting land uses). The PCA demonstrated that the main risk of groundwater nitrate pollution stemmed from the interaction between intrinsic vulnerability (related to hydrogeological factors and soil media) and land use; and this, together with groundwater flow, allowed us to explain the possible sources of nitrogen at each sampling point. The current demarcations of NVZs in central Spain reveal a lack of coordination between administrations and poorly defined criteria. Our results show the need to redefine these NVZs by mapping vulnerability to nitrate pollution within this territory based on the environmental factors highlighted by the PCA.